/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.instrument.classloading;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.List;

import org.springframework.instrument.InstrumentationSavingAgent;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

/**
 * {@link LoadTimeWeaver} relying on VM {@link Instrumentation}.
 *
 * <p>Start the JVM specifying the Java agent to be used, like as follows:
 *
 * <p><code class="code">-javaagent:path/to/org.springframework.instrument.jar</code>
 *
 * <p>where {@code org.springframework.instrument.jar} is a JAR file containing
 * the {@link InstrumentationSavingAgent} class, as shipped with Spring.
 *
 * <p>In Eclipse, for example, set the "Run configuration"'s JVM args to be of the form:
 *
 * <p><code class="code">-javaagent:${project_loc}/lib/org.springframework.instrument.jar</code>
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see InstrumentationSavingAgent
 * @since 2.0
 */
public class InstrumentationLoadTimeWeaver implements LoadTimeWeaver {

    private static final boolean AGENT_CLASS_PRESENT = ClassUtils.isPresent(
            "org.springframework.instrument.InstrumentationSavingAgent",
            InstrumentationLoadTimeWeaver.class.getClassLoader());


    @Nullable
    private final ClassLoader classLoader;

    @Nullable
    private final Instrumentation instrumentation;

    private final List<ClassFileTransformer> transformers = new ArrayList<>(4);


    /**
     * Create a new InstrumentationLoadTimeWeaver for the default ClassLoader.
     */
    public InstrumentationLoadTimeWeaver() {
        this(ClassUtils.getDefaultClassLoader());
    }

    /**
     * Create a new InstrumentationLoadTimeWeaver for the given ClassLoader.
     *
     * @param classLoader the ClassLoader that registered transformers are supposed to apply to
     */
    public InstrumentationLoadTimeWeaver(@Nullable ClassLoader classLoader) {
        this.classLoader = classLoader;
        this.instrumentation = getInstrumentation();
    }

    /**
     * Check whether an Instrumentation instance is available for the current VM.
     *
     * @see #getInstrumentation()
     */
    public static boolean isInstrumentationAvailable() {
        return (getInstrumentation() != null);
    }

    /**
     * Obtain the Instrumentation instance for the current VM, if available.
     *
     * @return the Instrumentation instance, or {@code null} if none found
     * @see #isInstrumentationAvailable()
     */
    @Nullable
    private static Instrumentation getInstrumentation() {
        if (AGENT_CLASS_PRESENT) {
            return InstrumentationAccessor.getInstrumentation();
        }
        else {
            return null;
        }
    }

    @Override
    public void addTransformer(ClassFileTransformer transformer) {
        Assert.notNull(transformer, "Transformer must not be null");
        FilteringClassFileTransformer actualTransformer =
                new FilteringClassFileTransformer(transformer, this.classLoader);
        synchronized (this.transformers) {
            Assert.state(this.instrumentation != null,
                    "Must start with Java agent to use InstrumentationLoadTimeWeaver. See Spring documentation.");
            this.instrumentation.addTransformer(actualTransformer);
            this.transformers.add(actualTransformer);
        }
    }

    /**
     * We have the ability to weave the current class loader when starting the
     * JVM in this way, so the instrumentable class loader will always be the
     * current loader.
     */
    @Override
    public ClassLoader getInstrumentableClassLoader() {
        Assert.state(this.classLoader != null, "No ClassLoader available");
        return this.classLoader;
    }

    /**
     * This implementation always returns a {@link SimpleThrowawayClassLoader}.
     */
    @Override
    public ClassLoader getThrowawayClassLoader() {
        return new SimpleThrowawayClassLoader(getInstrumentableClassLoader());
    }

    /**
     * Remove all registered transformers, in inverse order of registration.
     */
    public void removeTransformers() {
        synchronized (this.transformers) {
            if (this.instrumentation != null && !this.transformers.isEmpty()) {
                for (int i = this.transformers.size() - 1; i >= 0; i--) {
                    this.instrumentation.removeTransformer(this.transformers.get(i));
                }
                this.transformers.clear();
            }
        }
    }

    /**
     * Inner class to avoid InstrumentationSavingAgent dependency.
     */
    private static class InstrumentationAccessor {

        public static Instrumentation getInstrumentation() {
            return InstrumentationSavingAgent.getInstrumentation();
        }
    }


    /**
     * Decorator that only applies the given target transformer to a specific ClassLoader.
     */
    private static class FilteringClassFileTransformer implements ClassFileTransformer {

        private final ClassFileTransformer targetTransformer;

        @Nullable
        private final ClassLoader targetClassLoader;

        public FilteringClassFileTransformer(
                ClassFileTransformer targetTransformer, @Nullable ClassLoader targetClassLoader) {

            this.targetTransformer = targetTransformer;
            this.targetClassLoader = targetClassLoader;
        }

        @Override
        @Nullable
        public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
                                ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

            if (this.targetClassLoader != loader) {
                return null;
            }
            return this.targetTransformer.transform(
                    loader, className, classBeingRedefined, protectionDomain, classfileBuffer);
        }

        @Override
        public String toString() {
            return "FilteringClassFileTransformer for: " + this.targetTransformer.toString();
        }
    }

}
